(1) Field of the Invention
The present invention generally relates to a photo-semiconductor module for emitting a light beam, and more particularly to a photo-semiconductor module in which a light source is cooled by a thermo-element so as to be maintained within a predetermined temperature range.
A photo-semiconductor module for emitting a light beam is used for optical communication. The photo-semiconductor module generally comprises a photo-semiconductor element such as a laser diode (hereinafter referred to as an LD element), an optical system, a photo detector such as a photo diode (hereinafter referred to as a PD element) and the like in a casing. A light beam emitted from the LD element travels through the optical system and goes out of (i.e., exits) the casing.
To drive the LD element in a stable state, the LD element must be maintained within a constant temperature range even if environmental conditions of the LD element are varied. Thus, a thermo-element is mounted in the casing of the photo-semiconductor module. The thermo-element cools the LD element so that the LD element, which is driven, is maintained within a predetermined temperature range.
(2) Description of the Related Art
A conventional photo-semiconductor module is shown in FIG.1. Referring to FIG. 1, an LD element 3 is fixed on a first holder 8a, a lens 4 is supported by a second holder 8b and a PD element 6 is fixed on a third holder 8c. The first, second and third holders 8a, 8b and 8c are mounted on a base plate 10 so that the first holder 8a is positioned between the second and third holders 8b and 8c. The base plate 10 is mounted on a thermo-element 7 fixed on a bottom plate of the casing 2. The LD element 3 a emits laser beam from the front and from the rear surfaces thereof. The laser beam emitted from the rear surface of the LD element 3 is detected by the PD element 6. The LD element 3 is controlled, based on a detecting signal from the PD element 6, so that the output power of the LD element 3 is within a predetermined range. The laser beam emitted from the front surface of the LD element 3 travels through the lens 4 and a window 5, provided on a wall of the casing 2, and goes out of i.e., exits the casing 2.
The LD element 3, the lens 4 and the PD element 6 are respectively thermally coupled to the thermo-element 7 via the base plate 10 and the first, second and third holders 8a, 8b and 8c. The thermo-element 7 is supplied with an electric current and cools the LD element 3, the lens 4 and the PD element 6, based on a Peltier effect. That is, the thermo-element 7 operates as a cooling-element for the LD element 3. A thermo-detector 9 for detecting the temperature of the first holder 8a is fixed on the first holder 8a. The amount of electric current to be supplied to the thermo-element 7 is controlled, based on a detecting signal from the thermo-detector 9, so that the LD element 3 is maintained within a predetermined temperature range.
To emit a narrow laser beam, optical axes of the LD element 3 and the lens 4 must be precisely aligned with each other. In the above conventional photo-semiconductor module, the LD element 3 and the lens 4 are respectively mounted on the single base plate 10 via the holders 8a, 8b. Thus, it is not difficult to precisely align the respective optical axes of the LD element 3 and the lens 4 with each other. On the other hand, as only a part of the laser beam from the LD element 3 may be input to the PD element 6, it is unnecessary to precisely align the respective optical axes of the LD element 3 and the PD element 6 with each other. However, in the above conventional photo-semiconductor module, although the PD element 6 and the LD element 3 need not precisely face each other, the PD element 6 is also mounted on the same base plate 10 as the LD element 3 and the lens 4. As a result, a large number of components (i.e., the LD element 3, the lens 4, the PD element 6, the holders 8a, 8b and 8c and the base plate 10) are thermally coupled to the thermo-element 7. Thus, to cool the large number of components so that the temperature thereof is controlled within a predetermined range, a large amount of electrical current must be supplied to the thermo-element 7. That is, the power dissipation of the thermo-element 7 becomes large.